Typical gas turbine engines include a compressor, a combustor, and a turbine. The sections of the gas turbine engine are sequentially situated about a longitudinal axis and are enclosed in an engine case. Air flows axially through the engine. As is well known in the art, air compressed in the compressor is mixed with fuel, ignited and burned in the combustor. The hot products of combustion emerging from the combustor are expanded in the turbine, thereby rotating the turbine and driving the compressor.
Both the compressor and the turbine include alternating rows of stationary vanes and rotating blades. The blades are secured within a rotating disk. The vanes are typically cantilevered from the engine case. The radially outer end of each vane is mounted onto the engine case at a forward attachment point and a rear attachment point.
It is critical that the vanes and blades do not come into contact with each other during engine operation. Even if one vane obstructs the rotating path of a blade during engine operation, the entire row of blades will become dented, bent, or damaged as a result of the high rotational speeds of the blades. Even relatively small damage on the blade will propagate as a result of the centrifugal forces to which the rotating blades are subjected. Ultimately, this will result in the loss of a blade or a part thereof. Furthermore, damage disposed on the radially inward portion of the blade is more undesirable since the greater centrifugal force increases the likelihood of failure.
Axial clearance between the rows of vanes and blades is provided to prevent interference between the stationary vanes and the rotating vanes. For optimal gas turbine engine performance, it is desirable to minimize axial clearance between the blades and vanes. However, axial clearance must be sufficient to avoid the risk of potential interference between the vanes and blades.
A number of factors contribute to risk of interference between vanes and blades. One factor affecting the axial clearance is future wear resulting from normal operating life of the gas turbine engine. The normal wear loosens the fit between the parts of the engine and allows additional axial movement therebetween. Axial movement resulting from future wear dictates a larger axial clearance than is desirable in order to compensate for any such future wear.
Another factor contributing to risk of interference between vanes and blades is the different rates of expansion of the engine case. The engine case is fabricated from metal and includes portions of varying thickness. During the transient conditions of engine operation, the different portions of the engine case heat up at different rates. The thinner portions heat and thermally expand faster than the thicker portions. The thickness of the engine case at the forward attachment point of the vane is greater than the thickness of the engine case at the rear attachment point of the vane. Therefore, while the forward attachment point expands relatively slowly during transient conditions, the rear attachment point expands relatively quickly. With expansion of the rear attachment point area, the rear portion of the vane, also known as the trailing edge, moves radially outward, while the front portion of the vane, known as the leading edge, remains substantially stationary. Such movement of the radially outer diameter portion of the trailing edge of the vane tilts the radially inner diameter portion of the vane towards the blades, thereby reducing the axial gap between the blades and vanes and threatening to cause blade damage on the radially inner portion thereof.
Currently, such axial spacing concerns are addressed by tight dimensional tolerances. Initial axial clearance tends to be larger than desired to account for different expansion rates of the engine case and to anticipate any future wear. Additional axial clearance makes sealing between static and rotating structure more difficult, adds extra weight, and has a negative impact on the aerodynamics of the gas turbine engine.
One approach to reduce risk of contact between the vanes and the blades is to increase thickness of the engine case in the thinner portions thereof, so that the rate of thermal expansion is substantially the same throughout the engine case. However, the resulting extra weight adversely affects the overall efficiency of the gas turbine engine. Furthermore, in older engines, if wear erodes the mating parts of the engine case and vanes excessively, the entire engine case must be replaced, because it is impossible to add thickness to an existing engine case. Replacement costs of the engine case are extremely high.